AMP-activated protein kinase (AMPK) acts as a metabolic master switch controlling both fatty acid and glucose metabolism in mammalian cells. Recently, emerging evidence suggests that AMPK regulates sustrate metabolism in striated muscle in response to altered energy supply or demand. AMPK is a heterotrimeric protein consisting of one catalytic subunit (alpha) and two regulatory subunits (beta and gamma). AMPK activity is regulated by both allosteric and covalent mechanisms. It is allosterically activated by an increase in AMP and inhibited by ATP and phosphocreatine. Phosphorylation by an upstream kinase, AMPK kinase, also activates AMPK. In the heart and skeletal muscle, increased AMPK activity has been shown during ischemia or hypoxia in which substantial depletion of ATP occurs (ref). However, activation of AMPK in skeletal muscle also occurs during exercise when (ATP) is minimally changed while the energy reserve compound, (PCr), is markedly decreased. These results suggest that depletion of energy reserve is sufficient to activate AMPK. Hypertrophied and failing hearts are characterized by chronic depletion of energy reserve, i.e. markedly lower (PCr) but near normal (ATP). These hearts also exhibit increased glucose utilization (ref). We have recently made the exciting observation that AMPK activity is dramatically increased in the hypertrophied rat heart due to chronic pressure overload of the left ventricle (LVH). This is the first observation that AMPK activity is increased in a chronic disease model with impaired myocardial energetics. Distinct from acute activation of AMPK reported in previous studies, we found that increased AMPK activity in LVH hearts was accompanied by altered expression of the alpha1 and alpha2 catalytic subunits of AMPK, specifically, upregulation of alpha1 and downregulation of alpha2. These findings, although preliminary, provide a basis for defining the functional significance of AMPK signaling in hearts with chronic alterations of myocardial energetics. In this proposal we will test the hypotheses that chronic depletion of energy reserve in hearts due to pressure overload alters the expression pattern and activity of AMPK, and activation of AMPK acts as a compensatory mechanism to maintain energy supply by increasing myocardial glucose utilization in hypertrophied hearts.